Welding of pipes made of thermoplastic materials generally leads to thickening of the walls of the pipes in the vicinity of the weld, and produces a lip or internal bead at the point of the join. This thickening of the wall or lip causes a reduction in the internal diameter of the pipe at the point of the weld, constricting the flow of fluid through the pipe. The internal lip or bead also creates a point where precipitates such as like lime scale or other insoluble materials can accumulate. A similar problem is encountered in cases in which the pipes are supported by an insert that stiffens the pipes and holds them in place. Typical inserts are made of plastic and frequently include a tab or flange against which the pipe ends are placed. These types of inserts remain in place after the weld is complete, thus constricting the pipe at the point of the join.
There has been some effort to produce inserts made of water-soluble materials. Use of such an insert would have the advantage that flowing water through the pipe after welding will wash away the insert, leaving an unconstricted join.
British Pat. No. 709824 discloses a method of butt-welding thermoplastic pipes that comprises inserting into the interior of the pipes at the point of the join a non-toxic material that dissolves without leaving any sediment. The materials disclosed include a soluble carbohydrate such as sugar or starch to which a soluble binder such as albumin or glucose may be added, or a natural earth such as Fullers' earth with a soluble binder. The linings are produced by extrusion or molding.
British Pat. No. 858517 discloses a lining sleeve for use in welding together abutting ends of tubular metal members. Two water-soluble compositions for manufacture of the sleeve are disclosed. One comprises of a water-soluble material made of water-free 20-60% NaCl, 60-20% MgCl2, and 20% Na2SiO3, and the other comprises 30% KAl(SO4)2.12H2O, 30% H3BO4, 10% CaCO3, and 30% Na2SiO3.
U.S. Pat. No. 4,228,941 discloses a method for joining by explosive welding a smaller inner metal pipe to a larger outer metal pipe that surrounds it. The method comprises inserting a rigid contact body made of a soluble composition comprising NaNO2, NaNO3, and KNO3.
The compositions known in the art for forming water-soluble inserts are not ideal. For example, they tend to comprise organic materials or environmentally unfriendly salts. In addition, many of the compositions known in the art comprise hygroscopic materials. Compositions that comprise a large concentration of a hygroscopic salt such as MgCl2 tend to absorb moisture, to agglomerate or to crumble, and inserts formed from such compositions tend to be weak and fragile. An insert for use in the joining of two pipes in which the insert prevents the introduction of welding material into the pipes and is made of an environmentally-friendly non-hygroscopic inorganic material that has the desired physical properties of being castable into an insert, sufficient heat resistance to be usable in welding, and high water solubility thus remains a long-felt but as yet unmet need.